Flattening an outer panel surface of a color cathode ray tube makes it possible to improve viewability of on-screen picture images. However, in Braun tubes of the shadow mask type, it is required that the effective surface section of a shadow mask be curved to a specified curvature in order to retain the required mechanical strength of such shadow mask. To this end, a scheme is known for adding a curvature to the inner surface of a panel opposing the shadow mask while at the same time flattening the outer panel surface. When the glass thickness at the panel periphery is greater than that at the center portion of the panel, however, the following problems tend to occur: (1) the resultant brightness or luminance intensity can decrease at peripheral portions of the display screen, and/or (2) the ease of manufacture of the panel can be reduced. Accordingly, in cases where the outer panel surface is made flat, it is not permissible to significantly enlarge the curvature of the inner panel surface. In this case, it is impossible or at least very difficult to employ a shadow mask which exhibits any sufficient curvature, which results in the occurrence of a problem as to unwanted reduction in the mechanical strength of the shadow mask.
One prior known approach to improving the strength of a shadow mask with less curvature (with an increased radius of curvature) is to employ a technique for forming beads or alternating irregular bent/curved portions within the effective surface area of such shadow mask, as disclosed for example in U.S. Pat. No. 5,506,466. This prior art approach has the inherent problem that the beads and/or irregular curved/bent portions of the shadow mask badly affect the image display to cause a visually appreciable projection onto the phosphorus layer or phosphor screen associated therewith.
U.S. Pat. No. 4,136,,300 discloses a panel structure having inner and outer surfaces that are flat, which employs a technique for giving a shadow mask a curvature by varying the pitch of such shadow mask. This US patent, however, fails to provide teachings as to a panel structure that includes an in-line type electron gun and a dot type phosphor screen. Additionally, while this document sets forth therein a method of forming a curvature for a shadow mask in case an inner panel surface is made flat with a slot type shadow mask, it involves no discussion of the problems occurring when the panel is reduced to practice or to any detailed arrangement thereof.
A technique for changing the electron beam spacing (known as xe2x80x9cS-sizexe2x80x9d among those skilled in the art) with a change in deflection angle in order to let the curvature of such shadow mask be greater than that on the inner panel surface has been disclosed in the International Display Workshop (IDW) 1998 at pp. 413-416. This technique as taught thereby does require the use of more than one electromagnetic quadrupole for causing the S-size to vary with a change in deflection angle.
U.S. Pat. No. 5,479,068 discloses a Braun tube with an inline type electron gun assembly and a dot type phosphor screen, which incorporates a technique for enlarging the longitudinal pitch values of shadow mask apertures or holes to thereby improve the color purity tolerances, while simultaneously increasing the electron beam transmissivities. Regrettably, U.S. Pat. No. 5,479,068 is silent about any curved shadow mask designs and also strength retainment schemes relating thereto.
The present invention is directed to a specific color cathode ray tube (color picture tube) of the type which includes a substantially flat outer panel surface with an inner panel surface and a shadow mask having a curvature, and is aimed at improvement in mechanical strength at or near a short or xe2x80x9cminorxe2x80x9d axis by letting the curvature of the shadow mask be large at a portion which is adjacent to the minor axis in particular.
To this end, the color cathode ray tube incorporating the principles of the invention is specifically arranged so that, in combination with an in-line type electron gun assembly and a dot type phosphor screen, a horizontal direction pitch value at upper and lower portions at or near the minor axis of a shadow mask is greater by five percent (5%) or above, preferably 10% or more, than a horizontal direction pitch value at the center of the shadow mask, while at the same time causing a ratio of the shadow mask""s vertical pitch to the horizontal pitch thereof to be less in value at the upper and lower portions in close proximity to the shadow mask minor axis than at the center of the shadow mask.